1. Field of the Invention
This invention relates to a method of producing recording sheets. More particularly, the invention relates to a method of producing recording sheets carrying thereon a color developer capable of coloring on contact with an almost colorless coloring compound (hereinafter, referred to as a color former).
2. Description of the Prior Art
Hitherto, recording materials utilizing the coloring reaction of a color former and compounds (color developers) which cause coloring when brought into contact with the color former are well known. (Examples of the color developer are clayish materials such as acid clay, active clay, attapulgite, zeolite, bentonite, kaolin, etc.; organic acids such as salycilic acid, tarnic acid, gallic acid, a phenol compound, etc., as well as the metal salts of these organic acid compounds; and acid polymers such as phenol formaldehyde resin, etc.). The color former is a material which forms color by donating an electron or by receiving a proton and the color developer is a material which receives an electron or donates a proton.
As recording materials based on the above-described phenomenon, there are pressure-sensitive copying papers as disclosed in, for example, U.S. Pat. Nos. 2,505,470; 2,505,489; 2,550,471; 2,548,366; 2,712,507; 2,730,456; 2,730,457; 3,418,250; and 3,896,255; and heat-sensitive recording papers as disclosed in, for example, Japanese Patent Publication No. 4160/68 (corresponding to U.S. Ser. No. 512546 now abandoned) and U.S. Pat. No. 2,939,009. Furthermore, a printing system is also known in which a colored image or record is obtained by supplying an ink containing a color former onto a recording sheet having coated thereon a color developer through a medium such as stencil as described in, for example, German Offenlegungsschrift 1,939,624. In the case of utilizing the above-described phenomenon in the aforesaid recording or copying systems, the application of local pressure such as by pencil, ball pen, or type writer, the application of heat, or other proper treatment for bringing the color former into contact with the color developer on the recording sheet.
The most typical embodiment of recording materials of this type is a pressure-sensitive copying paper. The color former containing layer for the pressure-sensitive copying paper is prepared by dissolving a color former in a solvent, dispersing the solution in a binder or encapsulating the solution in microcapsules, and then coating the dispersion or the microcapsule-containing coating composition on a support such as paper, plastic film, resin-coated paper, etc. On the other hand, a color developer-containing layer is prepared by dissolving or dispersing a color developer in a medium such as water together with a binder and coating the solution or dispersion on a support.
In general, the color former layer and the color developer layer are formed on the same side of a support or the opposite side of a support respectively, or on the surfaces of different supports separately.
In case of employing microcapsules, the color former in the microcapsules is released by the application of local pressure by hand writing or typewriting to come into contact with the color developer layer, whereby the record in conformity with the local pressure applied is obtained.
Various kinds of color developers are known as described above. However, when active clay, acid clay, etc., which is most frequently used at present, is used as the color developer, there is a difficulty that the colored material or image formed thereon is inferior in light resistance or water resistance. Also, when a phenol monomer or a phenol-formaldehyde resin is used as the color developer, there is also a difficulty that the coated surface is liable to cause yellowing.
The inventors have found that polyvalent metal salts of aromatic carboxylic acid are effective color developers having no such difficulties (see, U.S. Pat. No. 3,934,070).
That is, when the polyvalent metal salts of an aromatic carboxylic acid are used as the color developer, there are such merits as: (1) the colored material or image formed is superior in light resistance, (2) the colored material or image does not fade away by the action of water, and (3) when the coated layer of the color developer is exposed to light, it is slow to yellow, but there are yet several points which can be improved upon.
For example, a coating composition containing the polyvalent metal salt of an aromatic carboxylic acid tends to cause undesirable aggregating or increase the viscosity of coating compositions based on latex binders. Also, in this case, it is difficult to obtain a sufficient surface strength of the coated layer. Still further, even if a coating composition of a polyvalent metal salt of an aromatic carboxylic acid which does not cause the increase of viscosity is used, the surface of the coated paper becomes rough and sufficient smoothness is not obtained. These drawbacks are considered to be due to the fact that the coating composition containing the polyvalent metal salt of an aromatic carboxylic acid provides a high concentration of metal ion when dried after coating which has a bad influence on a binder such as latex, etc., and causes locally aggregation.
As one solution to this problem it may be considered to use a water-soluble binder which is resistant to the metal ion but the use of such a water-soluble binder encounters inevitably such difficulties that the viscosity of the coating composition becomes high and the coated surface in inferior in water resistance.